The invention is directed to a method for the automated manufacture of wound electrical components by contacting thin insulated wires to terminal elements on the basis of laser welding.
A method of this type wherein the laser welding is undertaken indirectly by irradiation of a lamina placed onto the wire is disclosed by German Published Application 33 07 773, incorporated herein.
In electrical components such as, for example, in relays, coils, transformers and other components, the induction coils employed therein are becoming smaller and smaller within the framework of a continuing miniaturization. This means that the winding wires thereby utilized have a smaller and smaller diameter, this makes the contacting of the coil terminals more difficult. For example, the lacquer-insulated winding wires can have a diameter in the range from 30 through 100 .mu.m (0.03 through 0.1 mm).
Up to now, lacquer-insulated winding wires were usually contacted to coil terminals by resistance, ultrasound or laser welding. The softening of wire and terminal during the welding process is thereby a function of the temperature that is produced by energy application, heat capacity, and heat elimination. The various welding processes particularly differ in how the energy is supplied to wire and terminal. Heat capacity and heat elimination of wire and terminal, by contrast, are independent of the welding process--in a first approximation--and are respectively lower at the winding wire than at the terminal.
In resistance welding wherein both electrodes contact the winding wire, the heat generating occurs in the winding wire and not in the terminal onto which the heat is transmitted by thermal conduction. The chronologically earlier heating of the winding wire in comparison to the terminal is therefore caused by earlier application of energy to the winding wire, lower heat capacity, and lower heat elimination. The earlier heating and, thus earlier softening of the winding wire and the pressing power of the electrodes then lead to great deformation of the winding wire given little deformation of the terminal. In resistance welding, moreover, the electrodes in the case of miniaturized components such as, for example, SMD coils, must have such small dimensions that their service life is too short for automated manufacture. Further, the contamination of the electrodes by lacquer residues deriving from the winding wire is disturbing.
In ultrasound welding, the sonotron pressure not only produces the welding pressure but simultaneously causes the friction that generates the heat. Due to the lower heat capacity and the lower heat elimination, the wire softens earlier than the terminal. The ultrasound oscillation acting on the softened wire deforms the winding wire to an even greater degree than does resistance welding. In ultrasound welding of lacquer-insulated wires having a diameter below 0.4 mm, an additional cover lamina is welded on (sandwich technique) for mechanically reinforcing the weld, or the weld is covered with glue (see European Patent 0 200 014, incorporated herein). The mechanical reinforcement of the weld is required since the wire deforms greatly during welding and its mechanical stability thereby decreases. The electrical reliability of the winding contacting is likewise diminished due to the deformation.
In laser welding, German Published Application 33 07 733, incorporated herein discloses that copper wire microfinish enamel cannot be directly welded to the terminal element. Given direct irradiation of the winding wire by the laser beam, the wire would also have to absorb the energy that is required for softening the terminal. The wire, however, thereby melts or evaporates before the terminal softens, so that a usable winding contacting does not result. In order to avoid this, the laser energy is not directly supplied to the joining zone but to the surface of a lamina that covers the wire and the terminal (sandwich technique). In laser welding with a sandwich technique, the cover material softens first, then the winding wire and, finally, that part of the terminal to be contacted. The cover material is melted, flows over the wire to the terminal contact, completely dissolves the wire, and forms a welded bridge between wire and terminal. Analogous to a soldered connection, the boundary surface between melt and non-melted wire is critical for the contacting. Since the wire fully dissolves in an undesirable fashion, the boundary surface has approximately the size of the wire cross section. There is the risk of embrittlement at this boundary surface and the risk of an increase in the electrical resistance as a consequence of oxygen enhancement, pore formation, inadequate recrystallization, etc. The relatively small boundary surface yields a lower mechanical and electrical stability of the laser-welded connection in comparison to a soldered connection wherein the wire is embedded in the solder with an unmodified cross section.
Also included in the problem area of laser welding is that the lacquer of solderable copper lacquer wires (for example, on a polyurethane basis) does not complicate the welding or even assists it in that it acts as a fluxing agent, but the lacquer of the copper lacquer wires which are difficult to solder (for example, on a polyamide basis) and that are required for SMD coils, leads to poor welding results. By contrast to resistance and ultrasound welding, laser emission ultimately exerts no mechanical pressure onto the members to be welded. The members to be welded, however, must touch one another since a gap between the members to be welded deteriorates the welding results.